Sensor arrays, such as sensors installed along pipelines, highways, railroad tracks or utility lines, are typically designed to operate for long periods of time, often years, once they have been installed, with little or no maintenance. Many such arrays form computer networks of nodes that communicate wirelessly with each other and ultimately with one or more central controllers (base stations), such as to collect sensor data or meter readings and forward them to the base stations or set sensor parameters or control other remote devices in response to commands from the base stations. The nodes may be installed in difficult-to-access locations, making maintenance, such as battery replacement, difficult and expensive.
Radio transmitters and receivers (transceivers) in the nodes provide the wireless communication between the nodes and between the nodes and the base stations. These radios can consume significant amounts of electric power. However, each node typically has a very limited electric energy source, such as a battery that is expected to keep the node operating for years, possibly for the entire expected life of the node. In some cases, the nodes are powered by solar, thermal or other types of energy harvesters, whose outputs are relatively small.
Consequently, energy-efficient wireless communication protocols have been developed. Most such protocols employ low duty cycles, in which the radios are switched on only periodically and for relatively short periods of time. For example, according to the ContikiMAC radio duty cycling protocol, the radios are switched on about eight times per second. The amount of time during which a radio is on, compared to the amount of time the radio is off, is referred to as the radio's duty cycle. Duty cycles as low as about 0.6% are used. While low duty cycle protocols save energy, they require nodes to maintain well synchronized clocks, so the nodes can turn their radios on at the same time or nearly the same time as their neighbors. Synchronizing clocks among a large number of nodes can be quite expensive in terms of energy and computer resource consumption. Improved low duty cycle protocols are, therefore, desirable.